SPID#: 44 Since the rhesus monkey is widely used as an animal model for study of neural and behavioral aspects of age-related dementia, it is important to establish the frequency of impairment among various age groups for those cognitive domains subject to decline. Only then can it be determined whether particular biological characteristics are associated with increased risk of impairment, or whether treatments may have beneficial effects. The present study was designed to determine the prevalence of impairment on six cognitive measures and to develop a composite measure to serve as an index of global cognitive function. To address the first of these aims, we obtained normative performance measures for male and female rhesus monkeys below the age of 15 years on (1) the number of errors made during the acquisition phase of the Delayed Non-Matching-to-Sample task (DNMS), (2) the number of errors made during performance of the DNMS, (3) the memory span in the spatial condition of the Delayed Recognition Span Test (DRST), a test of short-term spatial memory, (4) memory span in a non-spatial condition of the same test, (5) the number of errors in Spatial Reversal, a test in which a positional preference must be reversed, and (6) number of errors in Object Reversal, similar to Spatial Reversal but is a discrimination between a pair of objects. Impairment on these tasks was defined as performance below the range among subjects aged 15 years or less. The youngest of the aged monkeys (19-24 years) displayed prevalence rates of impairment significantly different from zero on all tasks except the DRST (non- spatial condition) and displayed the highest rate of impairment in a task measuring spatial memory (DRST-Spatial condition). For two measures, DNMS acquisition and Spatial Reversal, we observed continuing decline as age increased beyond 24 years. We next employed principal components analysis to derive a composite measure of cognitive ability which can serve as a single global index of cognitive function. The derived component, termed the Summary Cognition Score correlates more strongly with age (r=-.74, n=53, p<0.001) than does any of the single cognitive tests upon which it is based, and therefore should serve as clear-cut summary measure in examining anatomical and physiological concommitants of cognitive decline.